The invention relates to a tunable magnetron comprising coaxial cathode and anode systems, defining therebetween an annular evacuated interaction space. The magnetron includes a tuning body which is rotatably supported by means of rolling bearings has an active portion influencing the tuning of the magnetron. This portion has a conductivity which varies circumferentially for producing a periodic varation of the tuning upon rotation of the body. The body, together with its bearings, is situated in a space communicating with the interaction space. A magnetic circuit comprising two pole shoes situated on opposite sides of the interaction space produces an axial magnetic field through the interaction space. The magnetic circuit is closed via the rolling bearing which is axially closest to the interaction space.
Such a magnetron is described in SE Pat. No. 191 373 corresponding to U.S. Pat. No. 3,343,031. It may for example be used to produce HF-pulses whose frequency vary from pulse to pulse. However, in many applications of such a magnetron it is desirable to be able to control the frequency for enabling transmission of pulses having accurately predetermined frequencies.
In particular when pulses having accurately predetermined frequencies are to be transmitted while the tuning body rotates, very severe requirements are laid upon the bearing arrangement of the body. Due to unavoidable time delays in the triggering circuits, the tuning frequency of the magnetron at the instant of transmission and hence the frequency of the transmitted pulse must be predicted a small time interval before the transmission instant, which is made on the basis of the instantaneous timing of the magnetron at the predicted instant and the variation speed of the frequency, i.e. the time derivative of the tuning curve. If the prediction is to be effected with high accuray, then it is a requirement that the tuning curve is very smooth, because each deviation from smoothness of the curve will result in a deterioration in conformity between the predicted transmission frequency and the actual transmission frequency. For achieving the desired effect, the bearings must show a very uniform friction and rolling resistance. Furthermore, the wear must be small for achieving a proper operation life and no wearing products should be allowed to be produced that could penetrate into the interaction space and deposit themselves on the active surface of the cathode. These requirements must be fulfilled in spite of very difficult operation conditions, inter alia involving that the bearings operate in vacuum and are furthermore exposed to a relatively strong static magnetic field and varying temperature conditions.
In a known construction of tunable magnetron, conventional ball bearings of steel are used for supporting the rotatable tuning body. As a result of the fact that the bearings operate in vacuum and at a raised temperature, they cannot be lubricated in the usual manner by means of oil. Due to the difficulties of getting effective lubrication, it has proved to be necessary to decrease the surface pressure, i.e. the load per ball, to a minumum and, in order to achieve this, to increase the number of balls to a maximum. In the bearings of the known magnetrons, the ball retainer ring has therefore been omitted and the balls then roll close to and in direct contact with each other in the space between the inner and outer bearing rings.
A drawback for steel balls is that, at least in the bearing lying closest to the interaction space, the balls are magnetized by the locally prevailing static magnetic field so that each ball forms a small dipole. These magnetic dipoles assume different positions relative to the magnetizing field at the same time as they rotate around their own axis and all the time also assume different mutual positions. This gives rise to mutual attraction and repulsion forces of a more or less random character between the balls. A certain observed lack of smoothness of the tuning curve for continuous rotation of the tuning body with a consequent frequency spread relative to the predicted frequency has been attributed to this phenomenon in the known magnetrons. Mutual attraction forces between the balls and between the balls and the rings furthermore cause "stick-slip"-effects, which have a negative influence on the operation life. A further drawback of steel balls is that their hardness decreases with temperature. This inter alia involves that the temperature during the evacuation process must be limited. Thereby the quality of the vacuum is also limited.